The AM Forum

I am generating a new board for this driver design. Attached are the original schematic and my copy of it into EasyEDA so that I can generate the PCB. Please look at it carefully (WA1GFZ ??) to see if I copied it correctly.

WA1GFZ - your PMs are blocked and there is no email address. I tried to respond to your PM. Hopefully you will want to contribute to this thread.

I have most of the footprints correct.

Questions -

50uf non-polorized caps are crazy expensive. I subbed two back to back 27uF electrolytics. Are there any special characteristics I should be looking at for the caps (low ESR, etc)?

The 1000uf at 450V Electrolytics are huge. Can I place them off board so that I can save space on the PCB? Should I add some smaller ones or more bypasses in addition to the off-board caps?

Are there any design updates that are not listed on the schematic?

BTW, found some nice 2.2uf coupling caps - Panasonic ECWFD2J1225J - in stock and $1.5 each...Put a trimmer in place of R26/R27NFB Ladder in rig

Yes, the beer can 1000uFd @ 450Vs can be off-board. I did it that way. Just keep the leads as short as possible. If you build the +- 300V supply in the same enclosure, the filter caps should be enough by themselves. I think the extra caps are a precaution against remote supplies having PS leads of several feet. This is more about suppressing glitches and spikes rather than AC hum filtering and regulation. IIRC, there is no PS current variation during modulation being in class A, so dynamic regulation is not an issue.

Speaking of RF bypassing, etc:

Yes, capacitor bypasses are always a good idea to sprinkle around. I also added ferrite cores, #63, to all input and output leads on the MOSFET driver. The ones that snap on work well. In fact I have about 30 of them all around the shack in critical low level audio and RF leads just to have a more stable environment. The old days of RF in the audio never plague me anymore. This can become a big problem when running a KW and using antennas with exposed antenna tuners, openwire or antennas in close proximity... as well as unbalanced antenna currents in coaxial feedlines, etc. Operating on the higher bands can also bring out issues. Once we cross 40M and higher, it's a different, squirrely, RF whirl.

Thanks for the pointer! That is the type of cap I would prefer to use. Let me know how big it is i.e. how much space do I need between holes. No specs on that site for me to pick the correct footprint.

I was planning to use a 1oz copper board, 1.6mil thick. Traces at 20Mil for signals, more for power. Was going to do a ground plane on the bottom. Minimize vias. Grounded mounting holes on all 4 corners.

Fets mounted under the board with access holes to allow them to be screwed to a heatsink. I have done this before and it works quite well. Easy to install.

Feel free to comment on the above.

Overall board looks like 100mm by around 175mm. A 2 layer board from Taiwan for Qty of 10 is $40. Plus shipping on a slow boat for $15 and DHL for $26.

Overall board looks like 100mm by around 175mm. A 2 layer board from Taiwan for Qty of 10 is $40. Plus shipping on a slow boat for $15 and DHL for $26.Not a bad price!

John

Hi John,

$40 + shipping for ten is a tremendous price. It's not even worth it to do point-to-point anymore. Wish they were available when I built mine.

Anyway, they will sell out quickly. This is the breakthru step between having average audio to transparant pristine audio. Especially when the audio negative feedback is added.

I was talking to Frank WA1GFZ and he said he will send you an email. He will answer your questions and also wants a board for his new heavily-modified Valiant project. Frank even wound his own FB mod xfmr.

Thanks for the excellent effort, John! " Chirp-chirp - AMers were only waiting for this moment to arrive..."

BTW, your Easy EDA schematic is MUCH easier to read than the original. Use that one only - once it is perfected. (Posted below)

It's important to point out that this driver uses only the famous and inexpensive 11N90 900 volt MOSFET as used in most class E rigs. A well-proven device.

I would keep the beer cans on the schematic since Frank intended that much capacitance there. You could make a note that these caps are not physically on the board but should have short leads to the board. They can be mounted on the power supply to double as filter caps - as long as the leads are short. Short means = 6-8" or less to the audio driver PCB?

Keep the board comfortably big enough to work on parts, remove them, etc. Also, sometimes parts change size in the future, so plan on roomier than too tight. And remember we are dealing with 600VDC. (+ - 300V) So keep the trace spacing wider rather than too narrow so not to end up with charred boards.

BTW, I used a Variac on my + - 300V supply. I found I could reduce it somewhat to just what was required to drive the tube modulators. Same results in class A, less heat. Still, the 11N90s barely get warm. In my case using tetrode configured 4-1000A modulators, I was able to back it down to +- 260 volts. Big low-mu triode modulators like 833As will require the most voltage.

Important: The driver board's MOSFET gate bias resistors are optimized for +- 300V. When the power supply voltage is changed too low, some of the FET gate bias resistors need to shift in value to get back to maximum output capability. So watch the scope carefully when reducing voltage.

How it works, simplified: The + - 300V supplies basically center tap and produce 0 volts to the tube grids thru the FETS. You set this DC offset that sets the negative bias and adjusts the idle current for the tubes. Then the audio signal comes thru push-pull and swings the grids up and down around this DC bias point. So if you need less than +- 300V of audio swing to fully drive the tubes, you can back down the overall supply somewhat. Put an audio tone thru at full RF and audio power and you can easily see this + - 300V voltage Variac adjustment threshold on the scope.

I don't have any more comments at this point. Let Frank look over the final schematic and PCB to see what he thinks...

Thanks John. Is a regulated supply needed? or perhaps just rectified, cap filter, and perhaps a current limiting resistor?

FB, Steve -

The more help the better.

The driver board runs class A, a steady ~40 mA draw per leg, (two legs) so any brute force type power supply will work fine. I believe the PS continuous rating should then be at least 100 mA since the two class A legs are running together. Let me axe Frank about his rating opinion. No need for voltage regulation.

I have just one meter on the audio driver board panel monitoring the drain current of R4... a 50 mA meter in series with R4. This is a steady state current of 40 Ma or so. I read the modulator tube's grid current separately in the mod deck, since this is a dynamic measurement with audio fluctuation.

Just a heads-up to those ordering parts: Frank, John and I have been emailing and making some minor value changes. A few errors were found in the transition from the prototype to the schematic.... The C12 0.47 uF NFB cap is now 300 pF. That's what I had in mine all along.

I am using the NP 20 uF cap in the audio input, so that still stands. John is considering different pot wattage ratings too.

John will be posting an updated schematic.

BTW, yes, the QIX negative peak limiter would be a nice addition. It's really only a few parts, so worth adding. The circuit can be found within his PWM modulator board. (We should ask for Steve's permission, of course)

** Another point: I noticed that I never employed a way to key the driver board on and off for T/R. I just leave it running and a step start relay disconnects the audio drive. I probably didn't want to have power supply pops by keying the +- 300V. Maybe we can find a way to bias the whole circuit off during receive to kill (or greatly reduce) class A current without causing audio pops.

*** UPDATE: It was decided to keep any negative peak limiting circuitry off the driver board. This would require low level op amps, low voltage and other complications. Keep NPL with the low level processing and leave the driver board as is...

We are doing some finalization on the board footprints and making sure the tracks and vias are large enough and well spaced enough to handle +/- 300V. Also adding off board pots to allow for fine tuning adjustment of bias on the final mosfets. Frank is re-running the models to insure we have this right for various voltage applications.

What we will probably do is a small run to build some prototypes then a larger run with errata changes applied from the prototypes to get the final boards right. With the low costs for PCB runs in China we don't have to be perfect on the first run, just close. Takes the pressure off. It is a different world WRT PCB manufacturing than when this was tried back in 2010/2011.

My guess is that the large input cap will allow for most any low impedance, one volt audio driver.

Frank optimized everything in simulation so I imagine all is well. He's gone a step further and is now using IMD simulation results to fine tune the board, looking for over -40 dB 3rd.

It should be quite the design when he's finished.

T

As did I, I simulated the stages as well in MatLab/Simulink.

I would assume one would want consistent -3dB down frequency responses at each stage so there are no bumps or dips in the overall response.

Addendum: The actual 3dB down plots show the following. What I had quoted earlier was the "breakpoint" frequencies:

According to the latest schematic - in the first stage, the low frequency 3-dB down frequency response is 7Hz, the low frequency frequency response of the second stage is 3dB down at 92Hz, and the low frequency frequency response of the third stage has a 3dB down point of 4.5Hz. So one could use C9 = C1 = 2.2 uF and C4 = 0.1 uF if one wanted a lowest frequency response of ~ 100Hz for the the first three voltage amplifiers.

The audio input impedance (resistive part) at P1, with the R47 4.7k, is 1.4k.

Missed that there is a $100 tooling charge. This puts the board at $25-$30 my cost and I have to front the full amount and hope I can sell all of them. Are these still interesting at that ballpark price? I see why folks use China so much.

Frank has done a lot of thinking and optimization on the board behind the scenes. The IMD and flat response look heavenly. He's also looking into balanced input. It's his best effort to date.

You too have done an outstanding job laying it out and rounding up the perfect parts. The pads are big and the traces are spaced wide for +- 300VDC. There's a lot to like.

I will also order a board even though I have the old point to point prototype running FB. The opportunity to get a board may not happen again, so hey, even if I build another 813 rig or give it to someone else later on, it will get used.

I see my prodding enticed you to build one too, Bob! You will be as happy as a clam, especially after adding in NFB to your 813 rig. ;D

Second quote came in $20 higher than Oshpark. Awaiting one more quote.

If anyone has a suggestion on US based FABs to try, I am game. I used 4pcb and Oshpark so far.

Thinking about a 50% deposit on the boards to help me defray the lump sum cost to kick off the process.

Comments? Also, this is a first pass board. No guarantees that there won't be a bypass or other type of mod to get them running. My boards usually come back clean and don't have any defects, but it can happen in a first pass effort. Take a close look and if you see anything, speak up.

*** Hold off one more day, John. I have a feeling there are a few more takers... *** ;)

Last call guys!

This is the critical part to building the ultimate tube modulator. Even if it sits for a year, you will have something that may not be available again.

It's a great design, top quality and above all, cheap. No transformers in your audio negative feedback loop. Rock solid IMD specs and ultra-flat performance! Just use a decent mod xfmr and robust tube modulators and youse gots it all.

Emails or PMs sent out to all folks that expressed an interest. Did not want to post my paypal info here. If I missed you then PM me or post here.

I ordered enough for the current needs plus about 10 more, so plenty for a while. I will need to sell a few more to break even but with Tom on the wagon hawking these wares, I am sure that will occur. ;D

I set the price at $35 including free first class shipping (USA only). I am willing to ship international but shipping will be extra.

Here is the BOM. You should be able to import this at digikey and build your cart. I did not spec the external pots or the external large caps. I need to work with Frank to nail down those specs, so maybe wait a day or two if you are going to place an order. If you have trouble with the digi site, let me know. There is a feature where I can share a cart with you.

You will need to find a heat sink. Frank envisions that this board will be mounted very near the modulator and that the pots will be mounted so it is easy to fiddle with them for calibration but then not touched. So they need to be accessible but not on a front panel. I have 2 options for the NFB pot, one on board and it can also be substituted for a off board equivalent.

Does it need an honest heat sink, or do you / Frank believe possibly chassis mounting the fets would be enough?--Shane KD6VXI

Shane,

From my real experience with the prototype: I use a small 6" X 8" heatsink with 1/2" fins. (underneath - not shown here) The FETs and sink barely get warm and I leave them keyed on all the time. (The 4-1000A modulator tubes get T/R keyed in the fil CT) My guess is you could get away with just a 1/8" aluminum plate. It will also depend on how hard you idle and bias it - 833As vs: smaller 811As, etc.

Big tubes, then use a heatsink - small tubes then just a plate will probably do.

They say the proof is in the pudding. For those of you wondering what the GFZ MOSFET audio driver sounds like, well here it is.

This is a recording taken from on the air transmitted on 3885 and received thru an R-390 at Jeff/ W2NBC's QTH. You're listening to my homebrew pair of 4-1000As - plate modulated by a pair of tetrode-connected 4-1000As using regulated screen and grid supplies. It uses broadcash iron and heavy negative feedback. The transmit audio rolls off sharply at about 5.5 KHz so the signal bandwidth is reasonable.

I have just an average voice but must say this is my best building effort ever. (To sound immodest) I went to the ends of the Earth to get this rig running as cleanly as possible. Listen to the highs and lows.. balanced and clean. Not easy to do in a big rig like this. For once I am satisfied. And, you can be too!!… with the World Famous WA1GFZ MOSFET audio driver! ;) ;D

Tom,Do you do anything with the audio into the driver? Or is your control entirely just cutting the HV to the mod tubes.John

John,

Partly. I use the sequencer unit using time delay key contacts going into the one volt audio input of the MOSFET driver. Last thing coming on and first thing turning off. No audio spikes or key up noise. I have many relays in the shack and two are very loud HV supply step start contactors, so I need to delay the audio until the very end of the sequence. Without sequencer keying, the audio clack! is ridiculous.

(I sequencer key the fil CT and the HV to the modulator tubes too, so three ways.)

The only other solution would be using vacuum relays which would cost a fortune to populate.

The reason you haven't heard me, I hate to say, is that I haven't been on the air for a few years... just testing here and there. I'm focused solely on helping others build their rigs - or I'm trading the Dow Index futures market. I'm a type A very focused person and have trouble spreading out my attention. I'm hoping when these new rigs (like yours) get going, I will MAKE the time.

You will hear me. I run a 2-el delta loop at 190' beaming SW. Calif is easy on 75M when the solar cycle is low.

The board is approximately 133mmx171mm. I used Precision Technologies or eproto.com. I did my layout on EasyEDA. I also use Elecrow for boards if I want to go to China Fab.

They had a $100 setup charge then a sliding scale based on number of boards and fab time. I chose my number based on how many solid orders I had at the time, a few more to sell to late comers and free ones to Frank and Tom for the immense help provided during layout. I also did not want to tie up my money on a huge order with doubts on if I would sell them all.

I then set my price accordingly. Won't get rich for sure but maybe $2 an hour wage for my time if I happen to sell all of them. But hey, when does a hobby ever break even?

I hear that there are some questions on how to mount the fets. I will work on that over the next few days. I don't yet have some of the larger FETs but i think I can show you using TO-220 fets and you will be able to figure it out. Here is a non-pic overview...

First step is to lay the board on the heat sink, and spot the six holes for the mounts. Once they are tapped or drilled, then you can mount it and spot the FET holes. If you are a good machinist, you can spot them all at the same time. I am not.

The FETs are mounted under the board, with the tabs flush to the heat sink. Don't forget silpads and insulators for the screws. Approx 18mm away from the hole, you bend the wires up 90 degrees. Then you poke them thru the board and bend them slightly. Then mount the board on the sink. You should see the fets thru the large holes. Screw them down. Then wiggle the pins to reduce stress on them and solder the 3 pins on the board.

From then on, you can take the screws out, remove the board, and remount it and it will fit just right.

Frank thinks 1/2 in or 3/8 in nylon spacers. I prefer 3/8 bit I don't have any fets to look at right now to see which one is best. The spacers should be available at any good hardware store.

They are going fast! To the west coast or east??? If I didn't know better, I would think your name was Bill Cullen "The Price is Right"! How many of us remember him?

Good job, and thanks for making these available. I have always wanted to try the WA1GFZ driver circuit, but I did not want to wire point-to-point. This should make a very professional looking implementation!

Since most of the purchasers are from the east of the country, I think we can safely say that we are “Redefining East Coast Sound”!

Right, Tom?

Wow, 3 left. They will go fast now that that's that on that!

Well, you might say we are doing our best to help out and point the way to better audio. :D

A lot still depends on what the MOSFET board is driving. If a tube modulator has non-linearities and/or if the mod transformer is not of hi-fi quality, then we still have bottlenecks. At least we are removing SOME of the distorsion with a clean audio driver and overall NFB.

That said, I know of many big plate modulated rigs on the air that are built well and have high quality modulation components. They sound very good. Many use tube audio drivers with inter-stage transformers or 8 ohm hi-fi amplifiers driving backwards connected speaker/output transformers. When these rigs receive the all-new, disco-duck GFZ driver board with audio negative feedback, they will sound even better. Perhaps cleanly pass 50-100 Hz triangle waves that they could not before... AND generate a cleaner signal on the air!(lower IMD)

Over the next few months I hope we hear at least 1/2 of these boards on the air. I'm sure there will be plenty of questions and optimizing work going on - here on the AM BB.

Now that we are officially into the construction phase, it's time to move on:

Taken from the board notes posted in this thread by John, the voltages on the MOSFETS below will give you a good idea if the board is operating correctly.

Connect two resistor loads to simulate the tube grids. (Set the bias to -5 volts as the 300 volts is reached.)

When first built and tested, bring up the -+ 300VDC very slowly. Sample the individual MOSFET voltage points below to make sure they are roughly tracking the 300VDC. Once up to 300 VDC these voltage readings should be close.

Do a touchy-feely of all parts to be sure nothing is overheating. Be sure to respect the +- 300V supply when feeling around. Remember that - + 300V = 600 VDC when across them both.

Run some audio tones thru. 20 HZ triangles are a tough test and should easily pass. Make sure everything is FB before hooking up the modulator tube grids.

** As stated below, when connected to the real tube grids, be careful not to fry them since this is a low source impedance driver. Be careful when applying negative feedback and the potential for the modulator and driver board to take off in audio oscillation when the NFB is overdone. After 10 years of actual use with the prototype hand-wired version, I have never blown a grid fuse nor fried any grids.

The first person (guinea pig) to get one going should post his results since these boards have not been tested yet. You may be able to save others some time. ;)

Refer to the notes for more details.

Thanks.

T

Additional info and WARNING:

“My design was modeled after the Gates quad 807 audio driver. The fifth stage was because Tom wanted a stage referenced to ground to make feedback easier. The 11N90 source impedance is a lot lower than an 807 cathode follower so you have to be careful not to fry the tube grids.”

[The grid fuses will help if they are "fast-blo" and are sized right]

Frank / WA1GFZ:

Troubleshooting and pre test data - I recorded the operating voltages at each FET in simulation. This is assuming the output bias is set at -5 volts and the power supply is +/- 300 VDC.

For this first shipment, you will either receive them today or got them yesterday. Please post your build progress!

Let me know if there were any issues with packaging. This is the first time I have tried the Mylar packing envelopes. They seem to be always good with stuff I get from China so decided to give them a try on boards.

A second shipment of 3 packs of 2 boards each went out on Friday for K3YA, W3GMS and KC9LKE. Should get them on Monday.

Here are all the docs. You can upload the xls to digikey and use the quantity and the supplier part number to build your cart. I tested it. I did not try at mouser but you should be able to use the mfg part number. You may have to sub parts if they don't carry that exact part. I used the digikey parts to make sure the footprints were adequate. If you sub parts, you are on your own with respect to them fitting ok.

Mount the large 10W resistors slightly off the board. Also mount R14/R16 slightly off the board.

You will need to order the bias pots separate. They were crazy expensive at digikey. You can buy good pots at the surplus dealers for much less.

with 5 FETs sharing a heatsink, is there any concern for one or more FETs heating up and changing bias and idling currents of other FETs on the same heatsink?

Hi Bob,

I suppose anything could happen and produce a thermal runaway situation if taken to an extreme.

But in normal operation all the MOSFETS run pretty lukewarm and are very stable with capacitive coupling between stages. Even if one FET were to create a problem I think the normal power supply fusing or the grid fusing might protect it from an extended heat up. But ya never know when it comes to voltage spikes, oscillations or other dangers to SS devices.

I have always been an extremist when it comes to circuit protection. My first class E rig before circuit boards were available had about 7 different sensors and shutdowns. It was foolproof even sensing input and output swr. Also had a diode on the heatsink for temp sensing, etc. Steve went on to build a nice all-inclusive protection design with Hall effect devices that did it all and more - and is still available today.

So in the case of the GFZ driver board, there's no reason why you couldn't add a temperature probe on the heat sink to sound an alarm above a certain temp threshold. I have them sampling warm air above the finals in all of my bigs rigs. I use a digital/LED Kiln temp controller module that can be easily programed.

These are the type I use for $29.50. It would be easy to implement in the driver board. The top readout is the actual measured temp and the bottom is the preset trigger alarm limit. Easy as pie to use. Comes with a sensor. (Our Chinese friends)

Another factor that determines thermal stability is the circuit topology. If several stages are direct-coupled, (Direct Current) then a change in bias in the driving stage will likely cause a more dramatic bias change in the driven stage.

The board we are looking at here has resistive voltage dividers setting the bias for each individual stage, and the output push-pull stage has the bias individually set for each device. All stages are capacitively coupled, passing only the AC audio signal, not the DC bias. This results in a very stable design, and therefore thermal runaway due to the common heat sink should not be a concern.

The same device types are used for the amplifier, phase splitter, and the output stages. The lower level input stages run at much less current with higher resistances than the output stages, further contributing to the reliability due to their conservative operation. Tom's success with the design for a decade speaks volumes!